Piezoelectric devices provided with a piezoelectric body, which has piezoelectric characteristics such that the piezoelectric body expands and contracts in accordance with an increase and a decrease in electric field applied across the piezoelectric body, and electrodes for applying the electric field in a predetermined direction across the piezoelectric body have heretofore been used as actuators to be loaded on ink jet type recording heads, and the like.
As piezoelectric body materials, there have heretofore been known composite oxides having a perovskite structure, such as lead zirconate titanate (PZT). The composite oxides having the perovskite structure are ferroelectric substances, which have spontaneous polarization characteristics at the time free from electric field application. With the conventional piezoelectric devices, ordinarily, an electric field is applied in a direction matched with a polarization axis of the ferroelectric substance, and a piezoelectric effect extending in the direction of the polarization axis is thereby utilized. (The technique for applying the electric field in the direction matched with the polarization axis of the ferroelectric substance will hereinbelow be referred to as the conventional technique 1.) Specifically, heretofore, it has been regarded to be important that material design be made such that the direction of the electric field application and the direction of the polarization axis may coincide with each other (i.e., polarization axis=direction of electric field application).
However, in cases where the aforesaid piezoelectric effect of the ferroelectric substances is merely utilized, a strain displacement quantity of the piezoelectric device is limited. Therefore, nowadays there is a strong demand for piezoelectric devices having an enhanced strain displacement quantity.
Also, with size reduction and weight reduction made in electronic equipment and enhancement of functions made in electronic equipment in recent years, there has arisen a tendency toward the reduction in size and weight of piezoelectric devices and enhancement of functions of the piezoelectric devices. For example, in the cases of the piezoelectric devices for use in the ink jet type recording heads, such that images having good image quality may be obtained, it has recently been studied to enhance array density of the piezoelectric devices. Further, such that the array density of the piezoelectric devices may be enhanced, it has recently been studied to reduce the thicknesses of the piezoelectric devices. However, in the cases of the piezoelectric devices having the reduced thicknesses, if a voltage is applied across the piezoelectric device in the same manner as that for the conventional piezoelectric devices, the applied electric field exerted upon the piezoelectric body will become high. Therefore, in such cases, if the same material design as in the conventional piezoelectric devices is made directly, a sufficient piezoelectric effect will not be capable of being obtained.
FIG. 11 is a graph showing piezoelectric characteristics of piezoelectric bodies constituting conventional piezoelectric devices. It has heretofore been known that the piezoelectric characteristics (i.e., the relationship between the applied electric field and the strain displacement quantity), which are obtained from the aforesaid piezoelectric effect of the ferroelectric substance, may be represented approximately by a curve Q (conventional technique 1) as illustrated in FIG. 11. The curve Q indicates that, as for the applied electric field range of zero to a certain applied electric field Ex, the strain displacement quantity increases linearly with respect to the increase in applied electric field. The curve Q also indicates that, as for the applied electric field range higher than the certain applied electric field Ex, the increase in strain displacement quantity with respect to the increase in applied electric field becomes markedly small, and saturation is approximately reached in strain displacement quantity.
Heretofore, the piezoelectric devices have been used with the applied electric field falling within the range of 0 to Ex, in which range the strain displacement quantity increases linearly with respect to the increase in applied electric field. (For example, Ex takes a value of approximately 5 kV/cm to approximately 100 kV/cm, depending upon the kinds of the materials of the piezoelectric bodies, and the maximum applied electric field takes a value of approximately 0.1 kV/cm to approximately 10 kV/cm, depending upon the kinds of the materials of the piezoelectric bodies.) However, as for the piezoelectric devices having the reduced thicknesses, in cases where the voltage is applied across the piezoelectric device in the same manner as that for the conventional piezoelectric devices, the applied electric field exerted upon the piezoelectric body becomes high. Therefore, in such cases, the piezoelectric devices having the reduced thicknesses are used with the applied electric field falling within the range of, for example, 0 to Ey, where Ey>Ex. In such cases, a substantial piezoelectric constant may be represented by the inclination indicated by the broken line Q′ in FIG. 11. Specifically, in such cases, the substantial piezoelectric constant is smaller than the piezoelectric constant with respect to the applied electric field range of 0 to Ex, and the piezoelectric characteristics, which the piezoelectric devices originally have, are not capable of being utilized sufficiently.
Particularly, in cases where the difference between the minimum applied electric field and the maximum applied electric field is set at a level identical with the level in the conventional techniques, for example, in cases where the piezoelectric devices are used with the applied electric field falling within the range of Ex to Ey, the piezoelectric devices are used within the applied electric field range, in which little strain displacement is capable of being obtained, and therefore sufficient functions of the piezoelectric devices are not capable of being utilized.
In view of the above circumstances, a piezoelectric device, in which the piezoelectric body is caused by electric field application to undergo phase transition, has been proposed in, for example, Japanese Patent No. 3568107. (The piezoelectric device proposed in, for example, Japanese Patent No. 3568107 will hereinbelow be referred to as the conventional technique 2.) Japanese Patent No. 3568107 discloses a piezoelectric device comprising a phase transition film, electrodes, and a heating element for adjusting the temperature of the phase transition film at a temperature T in the vicinity of a Curie temperature Tc. (Reference may be made to Claim 1 of Japanese Patent No. 3568107.) Also, in Japanese Patent No. 3568107, as the phase transition film, there is mentioned a film, which undergoes the transition between a tetraqonal system and a rhombohedral system, or the transition between the rhombohedral system or the tetragonal system and a cubic system. (Reference may be made to Claim 2 of Japanese Patent No. 3568107.)
In Japanese Patent No. 3568107, it is described that, with the piezoelectric device in accordance with the invention of Japanese Patent No. 3566107, a strain displacement quantity larger than with conventional piezoelectric devices is capable of being obtained by virtue of a piezoelectric effect of a ferroelectric substance and a volume change due to a change of a crystal structure accompanying the phase transition.
In Japanese Patent No. 3568107, as the phase transition film, there are mentioned the film undergoing the phase transition between the tetragonal system and the rhombohedral system, each of which constitutes the ferroelectric substance, and the film undergoing the phase transition between the rhombohedral system or the tetragonal system, which constitutes the ferroelectric substance, and the cubic system, which constitutes a paraelectric substance. However, the piezoelectric device (the conventional technique 2) described in Japanese Patent No. 3568107 is the one used at the temperature in the vicinity of the Curie temperature Tc. The Curie temperature Tc corresponds to the phase transition temperature between the ferroelectric substance and the paraelectric substance. Therefore, in cases where the piezoelectric device is used at the temperature in the vicinity of the Curie temperature Tc, the phase transition film will not be capable of undergoing the phase transition between tetragonal system and the rhombohedral system. Accordingly, the piezoelectric device described in Japanese Patent No. 3568107 will be the one utilizing the phase transition between the ferroelectric substance and the paraelectric substance. With the piezoelectric device utilizing the phase transition between the ferroelectric substance and the paraelectric substance, since the paraelectric substance does not have the spontaneous polarization characteristics, after the phase transition has occurred, the piezoelectric effect extending in the direction of the polarization axis will not be capable of being obtained from the electric field application.
The piezoelectric characteristics of the piezoelectric device described in Japanese Patent No. 3568107 may be approximately represented by the curve R (conventional technique 2) as illustrated in FIG. 11. In FIG. 11, as an aid in facilitating the comparison, the piezoelectric characteristics before the phase transition occurs, which piezoelectric characteristics are represented by the curve R, are illustrated as being identical with the piezoelectric characteristics before the phase transition occurs, which piezoelectric characteristics are represented by the curve Q corresponding to the aforesaid conventional technique 1 utilizing only the piezoelectric effect of the ferroelectric substance. The curve R indicates that, as for the applied electric field range before the phase transition occurs, the strain displacement quantity increases linearly with respect to the increase in applied electric field due to the piezoelectric effect of the ferroelectric substance. The curve R also indicates that, as for the applied electric field range of an applied electric field E1, at which the phase transition begins, to an applied electric field E2, at which the phase transition is approximately completed, the strain displacement quantity increases due to the change of the crystal structure accompanying the phase transition. The curve R further indicates that, as for the applied electric field range higher than the applied electric field E2, at which the phase transition to the paraelectric substance is approximately completed, since the piezoelectric effect of the ferroelectric substance is not capable of being obtained any more, the strain displacement quantity does not increase with further application of the electric field.
With the conventional technique 2, as in the cases of the aforesaid conventional technique 1 utilizing only the piezoelectric effect of the ferroelectric substance, if the thickness of the piezoelectric device is reduced, the piezoelectric device having the reduced thickness will be used with the applied electric field containing the range of the high applied electric field, in which range little strain displacement quantity is obtained, and the operation efficiency will not be capable of being kept high.
In view of the above circumstances, the primary object of the present invention is to provide a piezoelectric device, which is capable of reliably yielding a large strain displacement quantity and is capable of coping with reduction in thickness.
An other object of the present invention is to provide a method of actuating the piezoelectric device.
A further object of the present invention is to provide a piezoelectric apparatus, which is provided with the piezoelectric device and control means for controlling the actuation of the piezoelectric device.
The specific object of the present invention is to provide a liquid discharge apparatus utilizing the piezoelectric apparatus.